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Home Archived Aug 4, 2016

USGS Geology in the Parks

Field trip map 2; Baker River

FIELD SIDETRIP F - Ancient Rocks along the Park Butte Trail

Long history in a block

Most of Park Butte is an immense block of exotic gneiss in the Bell Pass melange. The interested rock-reader can best view these exotic rocks, called the Yellow Aster Complex, just below a high fresh rockfall scar on Survey Point.

blocks of Yellow Aster Complex
Blocks below Survey Point reveal history of Yellow Aster Complex.

Blocks from the rockfall are many and varied, but common is a light-colored, quartz-rich gneiss mixed with, or next to, layers of marble, which can be recognized the latter by its grey color, rough, etched-looking surface, and the fact that it is easily scratched with a knife. The gneiss and marble in many blocks are folded and distorted. Cross-cutting the gneiss and marble are a variety of igneous dike rocks, some white and coarse-grained, like granite, and some dark. The dark dikes are metamorphosed basalt, called greenstone. The gneiss layers commonly contain metamorphic minerals known to be mostly derived from shaley limestone.

These rocks and the marble indicate that the Yellow Aster gneisses here are metamorphosed from sedimentary materials that once accumulated in the ocean. Locally, the marble contains tiny grey, metallic-looking graphite flakes, the recrystallized carbon from plant or animal remains. Isotopic analyses of zircon crystals from related gneiss at Yellow Aster Meadows suggest that sediments were derived from an old continent with rocks that formed at least 1,500 million years ago (Precambrian). Zircons from the cross-cutting igneous dikes, on the other hand, indicate that the dikes crystallized about 350 million years ago (mid-Paleozoic), but after the marine limestones and shales were metamorphosed into marble and gneiss. The Precambrian zircons were probably sand grains, eroded from Precambrian rocks exposed in some ancient continent and then deposited sometime before 350 million years ago. Scott Babcock, professor at Western Washington University and long a devotee of North Cascade geology, proposes, with a smile of creative imagination, that these rocks began life in the ancient supercontinent of Pangea somewhere in the vicinity of today’s Australia.

blocks of Yellow Aster Complex
Formation of the Yellow Aster Complex.
On to Railroad Grade Moraine

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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